This invention relates to silicones, and more particularly relates to techniques for producing silicone films.
In a broad sense, silicones are a class of polymers which consist of a repeating Si--O backbone with organic functional groups attached to the Si via Si--C bonds. A more precise term for this class of polymers is polyorganosiloxanes. The most common polyorganosiloxane is a linear polymer with two methyl groups per silicon atom, resulting in poly(dimethylsiloxane), or PDMS.
Silicones occupy an intermediate position between organic and inorganic compounds, specifically between silicates and organic polymers. This dual nature gives silicones considerable flexibility, lending them to a wide range of applications. Indeed, bulk silicones are available in many forms, ranging from low viscosity oils to highly-crosslinked resins and rubbers; and are used in applications ranging from caulk and paint additives to electronics and electrical connection encapsulation. In the field of biomedical applications, silicones are one of the most widely studied biomaterials, having been employed for medical implants, joint replacements, heart valves, catheters, intraocular lenses, and a range of other biomedical applications. In general, silicones are found to be one of the most biocompatible, long-term medical materials yet developed.
There is considerable and increasing interest in the production of silicone films, and particularly silicone thin films, for a variety of real and potential applications not addressed by the bulk material. In the biomedical field, silicone films are important for providing adherent, conformal coatings on implantable devices having complex topologies and small dimensions. In other applications, silicone films are well-suited as protective coatings for optical devices, as well as films for permselective membranes, abrasion- and corrosion-resistant coatings, photolithographic photoresists, optical components, and films for environmental sensors, among other applications.
Investigations into the production of silicone films have primarily focused on various plasma enhanced chemical vapor deposition (PECVD) techniques. PECVD involves the formation of a polymer film on a surface exposed to a vapor phase precursor in a plasma state that is produced by, e.g., radio frequency (RF) excitation. Specifically, the plasma is a partially ionized precursor gas, typically formed by exposing a gaseous monomer to an electric field at a relatively low pressure, and consists of electrons, ions, free radicals, molecules in excited states, and photons of various energies. Reactive species are generated in the plasma by various charged particle interactions such as electron impact collisions. Under appropriate conditions, these reactive species polymerize and deposit on an exposed surface to form a polymer silicone film. A wide range of monomers and plasma conditions have been investigated for producing PECVD silicone films.
It has been generally accepted that electron impact events in the plasma are responsible for the initiation of monomer fragmentation processes which lead to PECVD polymer formation and film growth. It has further been established that the UV radiation from the plasma causes UV photolysis of monomer groups which enables an intermediate polymer chain growth process. Bombardment of the growing film by ions and UV irradiation from the plasma is also understood to effect film growth, specifically by creating active sites for film growth.
But electron impact fragmentation of a monomer gas in the plasma, as well as bombardment of a growing film with ions and with UV radiation from the plasma, are known to result in various unwanted atomic rearrangements in the film as well as trapping of defects in the film. One predominant defect is that of trapped free radicals, or dangling bonds. Upon exposure to atmosphere, such dangling bonds are oxidized, leading to concomitant changes in film structure and properties over time. This aging effect, known to be characteristic of PECVD polymer films, renders such films inadequate for many applications.
In addition, the ion bombardment that is characteristic of PECVD processes has the effect of increasing the crosslinking density of a PECVD polymer film over that of the conventional polymer. Increased cross linking density typically renders the film inflexible or even brittle. PECVD silicone films are also typically characterized by a high dielectric loss when compared with the conventional bulk polymer. Thus, although it has been established that ion bombardment and electron impact events play a role in the formation of a silicone film in a PECVD process, such are also found to result in degradation of film properties.